1. Field of the Invention
This invention relates to a secondary emulsifier and co-emulsifier for a synthetic polymer process.
2. Description of the Prior Art
Salts of condensation products of formaldehyde and naphthalenesulfonic acid have been used as a secondary emulsifier and as a co-emulsifier in the preparation of synthetic polymer latex via free radically initiated emulsion polymerization. Use of a secondary emulsifier in an emulsion polymerization system greatly enhances the stability of the latex during synthesis, i.e., eliminates or minimizes the formation of precoagulum during the reaction. A co-emulsifier such as a nonionic surfactant is used to stabilize the latex.
Formaldehyde condensed naphthalene sulfonates of the Lomar.RTM., Tamol.RTM., Daxad.RTM. and Darvan.RTM.--types have been used in emulsion polymerization as secondary emulsifiers since the adoption of GR-S recipes during World War II. Because of the uniqueness of this class of chemical compounds, no substitutes have been discovered for the past thirty years.
The primary function of a secondary emulsifier is to permit addition of electrolyte to a polymerization recipe without subsequent instability during reaction. Without a secondary emulsifier, a typical emulsion polymerization recipe, containing added electrolyte, produces an unstable latex.
As the conversion from monomer to polymer increases, destabilization, as evidenced by precoagulum formation, occurs without the aid of a secondary emulsifier. Elimination of electrolyte from the recipe results in unacceptable viscosity increases during the reaction. An electrolyte is necessary to maintain fluidity during the polymerization reaction. A secondary emulsifier is required to offset the deleterious side affects of the electrolyte.
The term secondary emulsifier is a misnomer as the formaldehyde-condensed naphthalene sulfonates are not emulsifiers according to the definition of the term. For example, Lomar.RTM. PW has no affect on lowering surface tension and would, therefore, have no definable (infinite) critical micelle concentration (C.M.C.). A formaldehyde-condensed naphthalene sulfonate is neither an emulsifier nor a surfactant. It behaves more like an in-situ protective colloid during polymerization. By not having surfactant characteristics, this sulfonate has no effect on particle size and particle formation during polymerization. This is why formaldehyde-condensed naphthalene sulfonates are unique. They are the only clsss of compounds that enhance latex stability without affecting particle size during emulsion polymerization. Formaldehyde-condensed naphthalene sulfonates are excellent products for this application.
A major drawback of formaldehyde-condensed naphthalene sulfonates has recently been emphasized. These sulfonates are not biodegradable. Major rubber companies, which use these sulfonates in polymerization, have been cited by the Environmental Protection Agency (EPA) for pollution of major waterways throughout the United States, primarily in the Gulf states. Formaldehyde-condensed naphthalene sulfonates are extremely difficult to precipitate from aqueous solutions as all common alkali salts as well as the free acid form are soluble in water. During rubber coagulation, whether by salt-acid, aluminum, calcium or magnesium salts, formaldehyde-condensed naphthalene sulfonates remain soluble in water and do not coprecipitate with the polymer. They, therefore, evolve as plant effluent into settling ponds. Water treating techniques are fruitless. Flocculents cannot be economically employed to precipitate formaldehyde-condensed naphthalene sulfonates. Aeration or oxidation is ineffective as these sulfonates are not susceptible to biological degradation. Formaldehyde-condensed naphthalene sulfonates have been cited as a major contributing factor in pollution caused by polymer plant effluent. Because of this there is a definite need for an alternate product which is functionally equivalent to formaldehyde condensed naphthalene sulfonates, but is either biodegradable or precipitatable.